Certain oligosaccharides which are indigestible by humans may nevertheless serve as food for probiotic microorganisms and promote human health. Such oligosaccharides are known as prebiotics. Oligosaccharides are oligomers of sugar units (monosaccharides) linked by glycosidic bonds with degrees of polymerization (DP) from 2 to 10 or in some cases up to 20. Examples of prebiotic oligosaccharides include fructooligosaccharides (FOS), galactooligosaccharides (GOS), xylooligosaccharides (XOS), isomaltooligosaccharides (IMO), and pecticoligosaccharides (POS). Passing through the upper gastrointestinal tract intact, prebiotic oligosaccharides can be selectively metabolized by the beneficial bacteria in the colon, thus modulating the composition and/or activity of the gut microbiota and resulting in improvement to host health. The direct physiological benefits encompass stimulation of probiotic population such as Bifidobacterium and Lactobacillus strains and accumulation of metabolic end products, such as short-chain fatty acid (SCFA) in the colon. Besides the direct benefits above to hosts, a range of systemic health implications are recognized, including metabolic inhibition of pathogenic microorganisms, constipation alleviation, reduction of diet-induced obesity, improvement of mineral absorption, repression of allergic symptoms, enhancement of immune system, reduction of colon cancer, and modulation of cholesterol levels. The bioactivities and prebiotic functionalities of oligosaccharides depend on their sugar compositions, DP value, and glycosidic linkages.
At present, prebiotics are generally produced through either controlled hydrolysis of polysaccharides or direct synthesis from simple sugars. Methods using controlled hydrolysis of polysaccharides are limited by the starting polysaccharides since sugar compositions and glycosidic linkages of the oligosaccharides are mostly inherited from the parent polysaccharides and their supplies are limited. Enzymatic hydrolysis can be expensive and suitable enzymes of limited availability. Acid hydrolysis is cheaper, but may often induce sugar degradation and result in undesirable side-products. Synthesis of oligosaccharides from simple sugars using enzymes is also an expensive and problematic process in view of the challenges of identifying and synthesizing enzymes with high activity and selectivity, good stability and recyclability, low cost, and industrial feasibility. Non-enzymatic catalysts, especially acids, can also catalyze the glycosylation. However, challenges remain to prepare oligosaccharides with the degree of polymerization (DP)>2 while avoiding sugar degradation and other undesired byproducts.